CA2027223A1 - Process for preparing copolymers of .alpha.-methylstyrene and acrylonitrile - Google Patents

Abstract

Abstract of the Invention Copolymers of alpha-methylstyrene and acrylonitrile having improved composition of uniformity are prepared by emulsion polymerizing a first monomer stream having a weight ratio of alpha-methylstyrene/acrylonitrile from 75/25 to 85/15 until the amount of unreacted alpha-methylstyrene plus acrylonitrile based on total monomer and polymer weight is 20-45 percent and the excess of unreacted alpha-methylstyrene monomer is from 670 to 5000 weight percent based on unreacted acrylonitrile, thereafter polymerizing a second monomer feed stream comprising acrylonitrile until the amount of unreacted alpha-methylstyrene plus acrylonitrile remaining in the reactor is 5-15 percent based on total monomer weight, and continuing the emulsion polymerization until total monomer conversion reaches at least 90 percent.

35,455A-F

Description

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PROCESS FOR PREPARING COPOLYMERS ~F ALPHA-METHYLSTYRENE
AND ACRYLONITRILE
The present invention relates to a process for preparing copolymers of alpha-methylstyrene and acrylonitrile. More particularly the present invention reIates to an emulsion process for preparing such copolymers having improved~uniformity of monomer content thereby producing polymers improved physica1 properties.
Batch, emulsion polymerizations o~ alpha-methylstyrene and acrylonitrile wherein both monomers are added initially or by means of one or two additions during the course of the the polymerization result in compositions having monomer contents that vary due to the variation in monomer content experienced during the polymerization. Such variation or compositional drift is the result of a disparity in the reactivity of the two monomers. In particular, it has been found that acrylonitrile~is more reactive under skandard polymerization conditions and is consumed faster Shan is alpha-methylstyrene monomer. Consequently, copolymers prepared from an initial charge of monomers or otherwise utilizing reaction conditions that are not designed to account for the differing reactivities of the two ~ , 35,455A-F ; -1-:

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monomers comprise a mixture of polymers having differing monomer content.
Optimal polymer properties are believed to result ~rom copolymers comprising alpha-methylstyrene and acrylonitrile having a weight ratio of polymerized monomers from 70/30 to 85/15. Copolymerîc products comprising even small amounts of polymer product of a composition outside of this range tend to be deficient in desirable physical properties such as toughness or impact strength.
In USP 4,526,928 there is disclosed a process for the preparation of copolymers of alpha-methylstyrene and acrylonitrile wherein the ratio of alpha methylstyrene to acrylonitrile is maintained at greater than 90/10 at all times during the polymerization. While such process results in the preparation of a copolymer having a high glass transition temperature, it has now been discovered that processes involving the polymerization of monomers maintainin~ such a high ratio of alpha-methylstyrene to acrylonitrile disa~vantageously are characterized by lengthy reaction times due to the excessive amount of alpha-methylstyrene which has a reduced reaction rate compared to acrylonitrile Consequently, in order to provide a commerically acceptable polymerization process it would be desirable to achieve a reduction of reaction time. Moreover, it has now been discovered that glass transition temperatures of polymers prepared in the a~orementioned manner are not necessarily better than those prepared utilizing reduced ratios and that the toughness of the resulting polymer (as measured by notched Izod impact values) is o~ten inferior. This undesired result is believed to be due to compositional 35,455A-F -2-: , . . : , . : ~ ~.
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drift, i.e. the preparation of small amounts of polymer having inferior properties due to variation in monomer ratio during the polymerization.
In USP 4,294,946 there is also disclosed a proce~s for the preparation of copolymers of alpha-methylstyrene and acrylonitrile. In the disclosed process the reaction is not terminated until conversion of monomer exceeds 99 percent~ In the process, alpha-methylstyrene is batch loaded and acrylonitrile is added to the reactor by a secondary addition step. Although the reference suggests that the process may be operated in a continuous manner~ no teachings as to the relative rates of addition of the various monomers for such a continuous process were suggested. Furthermore, the reference provided no teaching as to the existence and effect of compositional drift in the copolymers or for a method for alleviating the same.
It would be desirable if there were provided an impro~ed process for preparing copolymers of alpha-meth~lstyrene and acrylonitrile resulting in poLymers of improved uniformity and physical properties.
Summary of the ~nvention According to the present invention there is now provided a process for preparing copolymers of alpha-methylstyrene and acrylonitrile having an overall monomer weight ratio (alpha-methylstyrene/acrylonitrile) from a~out 70/30 to about 85/15 comprising the steps Gf:
1) charging a first monomer feed stream eompri~ing alpha-methylstyrene and acrylonitrile in a weight ratio of from 75/25 to 85/15, in a continuous or semicontinuous manner to a well mixed reactor operating 35,4S5A-F _3_ ~, ~

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under emulsion polymerization conditions and initially comprising from 0.05 to 5.0 percent by weight based on final copolymer weight of a polymeric seed latex, until the amount of unreacted alpha-methylstyrene plus acrylonitrile in the reactor is from 20 to 45 percent based on total monomer and polymer weight;

2) thereafter, continuously or semicontinuously charging to the reactor with mixing under emulsion polymerization conditions a second monomer feed stream, comprising acrylonitrile in the substantial absence of alpha-methylstyrene, until the amount of unreacted alpha-methylstyrene plus acrylonitrile in the rPactor is from 5 to 15 percent base~ on total monomer and polymer weight;

3) thereafter, continuing the emulsion polymerization until total monomer conversion reaches at least 90 weight percent; and 4) recovering the copolymer o~
alpha methylstyrene and acrylonitrile.
Copolymers prepared according to the above described process possess an improved uniformity of composition and a concomitant improvement in physical properties, particularly improved toughness as measured by Izod impact compared to polymers prepared without regard to the above constraints. Consequently, formulated copolymer products prepared utilizing the present invented compositions are able to achieve improved levels of performance without concomitant sacrifice of alternative physical properties. More particularly, melt flow rate that normally is adversely affected by increasing levels of rubbery additives, may 35,455A-F -4-.' : ' .`
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be maintained at elevated levels by utilizing reducedamounts of impact modifying additives due to the inherent improved toughness of compositions prepared according to the present invented process.
The process of the present invention utilizes known emulsion technology including the use oE
emulsifiers such as sodium lauryl sulfate, potassium oleate, sodium oleate, potassium or sodium salts of fatty acids, potassium or sodium salts of rosin acids and sodium alkylbenzene sulfonates. A particularly desired emulsifier is sodium n-dodecylbenzene sulfonate.
All ingredients employed in the polymerization are purified prior to use. Moreover, the reaction is conducted under an inert atmosphere such as nitrogen, preferably at a slightly elevated pressure from 5 to 20 psig (140-240kPA). The reaction ls conducted at elevated temperatures on the order of 35 to ~5C, preferably from 50 to 90C. Agitation is employed in order to maintain a homogeneous reaction mixture.
free radical initiator, such as the well known sodium peroxydisulfate or potassium peroxydisulfate initiators, may be employed. A molecular welght regulator such as a mercaptan or other suitable chain transfer agent may also be employed.
In a preferred embodiment of the present invention, alpha-methylstyrene and acrylonitrile are initially added in a monomer weight ratio of alpha-methylstyrene to acrylonitrile from 80/20 to 85/lS. While the reaction is preferably conducted in a continuous manner, a semicontinuous addition of monomers may also be employed. By semicontinuous addition is meant addition of the monomer charge in ali~uots 35,455A-F -5-. :- .
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followed by reaction periods on the order of about one tenth of the total reaction time or less.
Typically, alpha-methylstyrene and acrylonitrile are continuously added over a period of from l to 6 hours under polymerization conditions. As a general rule, the initial charging of a first monomer feed stream attains an excess of u~reacted alpha-methylstyrene monomer of from 600 to 500(), preferably 670 to 5000 weight percent based on unreacted acrylonitrile. Addition of acrylonitrile monomer only starts soon after cessation of comonomer addition and is preferably continued under polymerization conditions for l to 4 hours. Addition of acrylonitrile usually results in the lowering of total quantity of unconverted monomers, although there generally remains an excess of unreacted alpha-methylstyrene of from 600 to 5000, preferably 670 to 5000 weight percent based on the weight of unreacted acrylonitrile. After complete addition of monomers, the polymerization is generally allowed to continue for from lS minutes to one hour.
In a highly preferred embodiment, the initial monomer concentrations and the rate of addition of acrylonitrile monomer are adjusted so as to yield a polymer product having a monomer weight ratio (alpha-methylstyrene/acrylonitrile) from 75/25 to 78/22~ In order to achieve commercially practical reaction times, conversions of 90% to 96 percent based on total monomer weight are generally employed. The product is recovered by normal recovery techniques including steam stripping of the resulting mixture to remove volatile components, coagulation, and pelletizing of the resulting product.

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In applications wherein additional modification of impact properties is desired, a rubber such as a styrene- or styrene/acrylonitrile- ~rafted polybutadiene rubber or a styrene/butadiene copolymer rubber may be employed. In a preferred embodiment a rubber latex is added to the alpha-methylstyrene/acrylonitrile copolymer latex prior to coagulation and pelletizing thereof. The resulting rubber modified polymer product may be dried and employed as an injection molding resin.
As previously noted, the addition of acrylonitrile monomer in a continuous or semicontinuous manner after an initial period of copolymer formation is conducted in a manner such that the monomer concentration in the reaction mixture is maintained between the aforementioned limits. Addition of acrylonitrile in a too rapid manner results in compositional drift due to the preparation of copolymers containing excess amounts of acrylonitrile and having inferior impact properties.
The seed latex employed in the initial polymerization preferabIy comprises any compatible polymeric latex having a particle size from lO0 to l,000 Angstroms. Highly preferred are polymeric seed latices comprising polystyrene, polybutadiene, copolymers of styrene and butadiene, or copolymers of alpha-methylstyrene and acrylonitrile.
Having described the new invention the Eollowing examples are provides as further illustrative and are not to be construed as limiting.
Example l A one-gallon (0.004 M3), glass-lined latex 35,455A-F _7 -.
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~ti ',,i ~',; ~ j,,J r,,, ~q reactor was charged with 1147 g of deionized water, 107 g of polystyrene seed latex (400 Angstrom number average particle size) (30 percent active, thereby providing 3 weight percent seed based on final polymer weight), 0.3 g of disodium ethylene diaminetetraacetic acid, 1.0 g of sodium bicarbonate, and 20 g of 43 percent aqueous sodium n-dodecylbenzene sulfonate. The mixture was purged with nitrogen, and the reactor was then evacuated with a water aspirator three times at room temperature.
After the final evacuatîon, the reactor was pressurized with 10 psi (69 KPa) of nitrogen. The reactor was heated to 85C and stirred at 2000 rpm. An aqueous feed consisting of 141 g of water, 41.8 g of 43 percent aqueous sodium n-dodecylbenzene sulfonate, and 1.4 g of sodium persulfate and a monomer feed consisting of 750 g of alpha~methylstyrene, 187 g of acrylonitrile, (providing a ratio alpha-methylstyrene/acrylonitrile of 80/20) and 2.0 g of n-octyl mercaptan (first monomer feed stream) were started simultaneously and added at rates of 37.4 and 234.6 g per hour to the reaction, respectively. After four hours, addition of first monomer feed stream was halted. The percentage of unreacted monomers based on total monomers and polymer was 31.4 percent. The amount of unreacted alpha-methylstyrene compared to acrylonitrile was 2070percent.
A second monomer feed stream consisting of 41 g of acrylonitrile and 0.59 g of n-octyl mercaptan was started at a rate of 10.4 g per hour. The aqueous feed rate was charged at 7.8 q per hour. The additions of aqueous feed and second monomer stream were continued for four additional hours. Total monomer feed resulted in a ratio of alpha-methylstyrene/-acrylonitrile, of 35,455A-F -8-,.: ; , . : : ~ . , .

_9 _ 77/23. The amount of unreacted monomers compared to total monomers and polymer was 13.6 percent. The amount of alpha-methylstyrene based on acrylonitrile was 650 percent. The second monomer feed was then halted, the aqueous feed continued for an additional 15 minutes, and the mixture was retained at 85C Eor 15 minutes. An antifoaming agent (antifoam H-10, 15 percent active, available from Dow Corning Corporation) was added and volatile components were steam distilled from the mixture. The reaction went to 90 percent conversion.
The ratio of polymerized alpha-methylstyrene/acrylonitrile in the finished latex was 76/24~
Examples 2 - 4 and Comparative Additional latex samples were prepared substantially according to the techniques of Example 1 utilizin~ various ratios oE monomers in the first monomer feed stream. The latexes were blended with a bimodal styrene/acrylonitrile grafted butadiene rubber prepared according to the following rubber preparation.
~ubber Preparation A twenty-gallon (0.07 M3), glass-lined latex reactor was charged with 61,979 g of bimodal butadiene rubber latex (75 percent 1400 Angstroms, 25 percent-8000 Angstromsl 33 percent active). The mixture was purged with nitrogen, then evacuated with a water aspirator three times at room temperature. The reactor was heated to 80C and stirred at 150 rpm. An aqueous feed~
consisting oE 6618 g of deionized water, 22.1 g oE
sodium persulfate, and 529.4 g of aqueous 43 percent sodium n-dodecylbenzene sulfonate, and a monomer feed, 35,455A-F _g_ , .
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consisting of 8603 g of styrene and 2427 g of acrylonitrile, were started simultaneously and added to the reactor at 1792 and 2757 g per hour, respectively.
The additions were completed in four hours and the reaction product was retained at 80C for an additional 30 minutes. An anti-foaming agent (Antifoam H-10, 15 percent active) was added and 650 ml of volatile reaction by products were steam stripped from the mixture. The reaction ran to 92 percent conversion.
The blends comprising 15 percent by weight rubber are prepared by latex blending, coagulating, dewatering, and pelletizing the resin. Test specimens are prepared by injection molding the resulting products. Tests on such specimens are conducted according to ASTM standards. Resulting are contained in Table I.

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f ~ o ~ 3 By examining Table I it may be seen that compositions having improved toughness and equivalent or improved melt flow rates are prepared by observing the processing conditions specified for the present invention.

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Claims (4)

1. A process for preparing copolymers of alpha-methylstyrene and acrylonitrile having a monomer weight ratio (alpha-methylstyrene/acrylonitrile) from 70/30 to 85/15 comprising the steps of:
1) charging a first monomer feed stream, comprising alpha-methylstyrene and acrylonitrile in a weight ratio of from 75/25 to 85/15, in a continuous or semicontinuous manner to a well mixed reactor operating under emulsion polymerization conditions and initially comprising from 0.05 to 5.0 percent by weight (based on final copolymer weight) of a polymeric seed latex, until the amount of unreacted alpha-methylstyrene plus acrylonitrile based on total monomer and polymer weight is 20-45 percent and the excess of unreacted alpha-methylstyrene monomer is from 670 to 5000 weight percent based on unreacted acrylonitrile;

2) thereafter, continuously or semicontinuously charging to the reactor with mixing under emulsion polymerization conditions a second monomer feed stream, comprising acrylonitrile in the substantial absence of alpha-methylstyrene, until the amount of unreacted alpha-methylstyrene plus acrylonitrile remaining in the reactor is 5-15 percent based on total monomer weight;

35,455A-F -13-

3) thereafter continuing the emulsion polymerization until total monomer conversion reaches at least 90 weight percent; and

4) recovering the copolymer of alpha-methylstyrene and acrylonitrile.
2. A process according to Claim 1 wherein the first monomer stream has a weight ratio of alpha-methylstyrene/acrylonitrile from 80/20 to 85/15.
3. A process according to Claim 1 wherein the total monomer conversion reaches from 90 to 96 weight percent.

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